Spin trapping is a powerful and convenient technique for the study of free radical reactions. The breadth of applications ranges from clinical studies to high-energy physics. Over 1500 references to the technique have accumulated in Chemical Abstracts. STDBII, a spin trapping database, has been implemented on IBM PC/AT and MacIntosh personal computers. The package operates with no "add-ons." The program is powerful yet user- friendly; the command structure is similar to the familiar 1-2-3 light-bar menu; search strategy employs the method of Query-by-Example (QBE); logical combinations of any fields are accomplished by using AND, OR, NOR, and EXCEPT. Presently, STDBII (4.5) contains files for 5,5-dimethyl-1- pyrroline-N-oxide (DMPO), alpha-phenyl-N-tert-butyl nitrone (PBN), 2- methyl-2-nitrosopropane (MNP), alpha-(4-pyridyl-1-oxide)-N-tert-butyl nitrone (POBN), nitrosodurene (ND) and 3,5-dibromo-nitrosobenzene sulfonate (DBNBS). Data for other less popular traps are included in a catch-all file. Presently, the database files have more than 2000 references with over 8800 parameter entries. The STDBII files contain information on: 1) spin trap used; 2) radical trapped; 3) hyperfine splittings reported; 4) solvent; 5) g-value, if reported; 6) a terse description of how the radical was produced and observed; 7) full bibliographic data. The latest version (STDBII, 4.5) also contains kinetic data on spin trapping rates as well as a new parameter, the NoH value (the ratio of aN/aH), which has proved useful for the identification of DMPO adducts. STDBII helps researchers: 1) in identification of spin adducts from the sometimes unique hyperfine splitting parameters; 2) as a key to the spin trapping literature, and 3) as a vehicle to correct published errors. STDBII is now available to researchers both inside and outside NIEHS. The package includes a user manual that lists all of the compiled information on spin trapping. Scientists who do not presently have access to a computer can still benefit from STDBII because all of the database entries are printed in the STDBII User Manual.